About cipher block modes

Transcript

1. or: what the heck are those MCRYPT_MODE_ECB, MCRYPT_MODE_CBC constants? Block cipher modes woensdag 25 april 12
2. What are block cipher modes ‣ FOOTER TEXT ‣ Modes to handle “blocks” during block cipher encryption / decryption. ‣ Work on blocks of data (8-256 byte mostly) instead of a continuous stream. ‣ Each block is en/decrypted separately. ‣ mcrypt_*() functions in PHP woensdag 25 april 12
3. What are block cipher modes ‣ ECB - electronic cookbook ‣ CBC - cipher block chaining ‣ CFB - cipher feedback ‣ (N)OFB - Output feedback woensdag 25 april 12
4. Electronic Cookbook (ECB) http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation woensdag 25 april 12
5. Electronic Cookbook (ECB) ‣ ENCRYPT 10 EQUAL BLOCKS OF DATA <?php // The key size does not matter $key = "1234567890"; // Message is 10x the string HELLOYOU. Since each string is // 64bit, this will result in every HELLOYOU be encrypted // separately. $message = str_repeat("HELLOYOU", 10); // Blowfish is an encryption that uses 64bit blocks $crypted = mcrypt_encrypt(MCRYPT_BLOWFISH, $key, $message, MCRYPT_MODE_ECB); // Display the result in hex for ($i=0; $i!=strlen($crypted); $i++) { printf ("%02X ", ord($crypted[$i])); if ($i % 8 == 7) print "\n"; } woensdag 25 april 12
6. Electronic Cookbook (ECB) ‣ RESULT IS DETERMINISTIC 3F 89 AD 58 3C C8 21 CD 3F 89 AD 58 3C C8 21 CD 3F 89 AD 58 3C C8 21 CD 3F 89 AD 58 3C C8 21 CD 3F 89 AD 58 3C C8 21 CD 3F 89 AD 58 3C C8 21 CD 3F 89 AD 58 3C C8 21 CD 3F 89 AD 58 3C C8 21 CD 3F 89 AD 58 3C C8 21 CD 3F 89 AD 58 3C C8 21 CD woensdag 25 april 12
7. Electronic Cookbook (ECB) ‣ CREATE A CORRUPT ENCRYPTED BLOCK <?php // The key size does not matter $key = "1234567890"; // again: all padded to the blocksize $message = "1111111122222222333333334444444455555555666666667777777788888888"; // Blowfish is an encryption that uses 64bit blocks $crypted = mcrypt_encrypt(MCRYPT_BLOWFISH, $key, $message, MCRYPT_MODE_ECB); // Lets "corrupt" a byte in the second block $crypted[10] = "A"; // Decrypt, and see the results: $plaintext = mcrypt_decrypt(MCRYPT_BLOWFISH, $key, $crypted, MCRYPT_MODE_ECB); print $plaintext."\n"; woensdag 25 april 12
8. Electronic Cookbook (ECB) ‣ ERRORS ARE ISOLATED IN ONE BLOCK 11111111T#####zO333333334444444455555555666666667777777788888888 woensdag 25 april 12
9. Electronic Cookbook (ECB) ‣ PARALLEL ENCRYPTION AND DECRYPTION IS POSSIBLE Block 1 Block 6 Block 8 Block 2 Block 5 Block 4 Block 7 Block 3 Block 9 assemble Block 1 Block 2 Block 3 Block 4 Block 5 Block 6 Block 7 Block 8 Block 9 Thread 1 Thread 2 Thread 3 = woensdag 25 april 12
10. Cipher Block Chaining (CBC) http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation woensdag 25 april 12
11. Cipher Block Chaining (CBC) <?php // The key size does not matter $key = "1234567890"; // The IV MUST be equal to the block size of the encryption method $iv = "IAMWEASL"; // Message is 10x the string HELLOYOU. Since each string is // 64bit, this will result in every HELLOYOU be encrypted // separately. $message = str_repeat("HELLOYOU", 10); // Blowfish is an encryption that uses 64bit blocks $crypted = mcrypt_encrypt(MCRYPT_BLOWFISH, $key, $message, MCRYPT_MODE_CBC, $iv); // Display the result in hex for ($i=0; $i!=strlen($crypted); $i++) { printf ("%02X ", ord($crypted[$i])); if ($i % 8 == 7) print "\n"; } ‣ ENCRYPT 10 EQUAL BLOCKS OF DATA woensdag 25 april 12
12. Cipher Block Chaining (CBC) 02 67 2E AA 4A EB E1 C1 F8 DB A6 2A 66 47 22 A7 5A 5B 7B 46 7D 68 8E E4 B4 BE 7D F7 00 73 B0 DD 72 71 4D 32 A9 A2 36 73 BB 8E 42 25 49 1D 65 B6 D9 36 F2 43 6A A9 E2 85 E4 C0 56 CC 24 05 73 22 52 A3 BA 85 88 5C A3 0D 98 29 3F 87 15 76 2E 98 ‣ RESULT IS NON-DETERMINISTIC woensdag 25 april 12
13. Cipher Block Chaining (CBC) ‣ ERRORS ARE ISOLATED IN ONE BLOCK PLUS THE NEXT 11111111?Թ~*IU33&333334444444455555555666666667777777788888888 Limited error propagation. woensdag 25 april 12
14. Cipher Block Chaining (CBC) ‣ INCORRECT IV ONLY RESULTS IN FIRST BLOCK FAILURE +%,#&=#322222222333333334444444455555555666666667777777788888888 woensdag 25 april 12
15. Cipher Block Chaining (CBC) ‣ IV is not a additional secret key! ‣ non-deterministic, since we’re chaining each block ‣ Change IV for each message for optimal security for non- deterministic messages. woensdag 25 april 12
16. Cipher feedback (CFB) http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation woensdag 25 april 12
17. Cipher feedback (CFB) ‣ Only needs “encryption” ‣ Effectively convert a block cipher into a stream cipher. ‣ No padding is needed (can be used on non-matching block lenghts) woensdag 25 april 12
18. Output feedback (OFB) http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation woensdag 25 april 12
19. Output feedback (OFB) ‣ Don’t use MCRYPT_MODE_OFB (8bit) ‣ Use MCRYPT_MODE_NOFB ‣ Cipher text is fed back instead of the output. woensdag 25 april 12
20. Conclusion ‣ You should use MCRYPT_MODE_CBC. ‣ Use randomize IV’s for each message (mcrypt_create_iv()) ‣ You should use the correct cipher algorithm (DES vs AES) woensdag 25 april 12